A standard motor-vehicle power actuator has a housing, a normally reversible electric motor in the housing having an output shaft defining a shaft axis, an input gear fixed on the shaft axis and rotatable thereabout by the motor, a threaded spindle extending along and rotatable about a spindle axis in the housing, an output gear fixed on the spindle adjacent and out of contact with the input gear and coupled to the input gear, and a nut threaded on the spindle and nonrotatable relative thereto. Thus rotation of the spindle in one rotational sense displaces the nut axially relative to the spindle axis in an actuation direction and rotation of the spindle in an opposite rotational sense displaces the nut axially relative to the spindle in an opposite release direction.
As described in German patent 3,443,288 of J. Ingenhoven a coupling that can be overpowered is provided between the input gear and the output gear. Such a device is provided in a motor-vehicle door and is used to actuate its latch. A manual return device is provided to reset the actuator in its starting position by overpowering the coupling. Thus once the power actuator has released the latch, for instance, the user can return the latch to the ready position, repositioning the power actuator to its starting position. Unless the power actuator is manually reset, it cannot be used again to open the latch.
Accordingly in German utility model G 8,621,592 published 20 Nov. 1986 the motor loads a spring as it moves the nut into the actuated outer position. Once the motor is no longer energized, the stored spring force reverse rotates the spindle to return the parts to their starting position. This system does indeed reset itself, but is fairly bulky because the motor must be powerful enough to operate the latch and load the spring, and the spring must be powerful enough to reverse-drive the rotor of the motor and the entire drive train between the motor and the latch.